This disclosure relates to fuel cell separator plate assemblies and a method of manufacturing multiple separator plate assemblies and their flow field plates in a continuous, automated process.
A composition for and a manufacturing process to produce a monolithic fuel cell separator plate assembly has been developed, such as disclosed in United States Patent Application Publication No. 2010/0307681 and which is incorporated by reference. Thermally purified flake graphite and fluorinated ethylene propylene (FEP) resins materials, with a typical composition of 15-20% FEP, have been used to make the separator plate assembly. Other hydrophobic resins such as perfluoroalkoxy copolymer (PFA) and polytetrafluoroethylene (PTFE) are also suitable. The resins are available from manufacturers, such as DuPont.
The separator plate assembly is also known as a bi-polar separator plate and contains flow fields on both sides of the separator plate assembly that distribute the reactants within the fuel cell. One example flow field used in UTC Power PC-50 phosphoric acid fuel cells (PAFC) has a platform of 500 mm×500 mm and contains approximately 150 reactant flow channels that are about 1.4 mm wide by 0.7 mm deep for the cathode flow field and 1.4 mm wide by 1.0 mm deep for the anode flow fields. The over-all thickness of the anode flow field and cathode flow field is about 1.7 and 1.4 mm respectively.
Preforms are currently made in a batch process by depositing a powdered mixture of graphite-FEP into the molds. These powders have a bulk density of about 0.6-0.7 g/mL versus the molded density of 2.1-2.2 g/mL. There are several shortcomings to this dry process. It is difficult to achieve a uniform powder distribution across the part. This variation translates to poor uniformity in thickness and density across the part. There is the additional complication when one is trying to mold a flow field to net shape that the compression ratio for the powder over the web of the flow field is different than over the ribs of the flow field. This results in the ribs having a lower density than the web which result in lower thermal and electrical conductivity and higher acid absorption and acid transfer rates which are undesirable. The orientation of the graphite particles in the ribs tends to be more along the ribs than across the ribs which is also less desirable. The typical batch process used in manufacturing is quite expensive.
The separator plate assembly must have a very low electrolyte take-up and very low rate of thru-plane acid transfer over its 10-20 year life. This is the most demanding characteristic of the separator plate assembly. Acid permeates the separator plate assembly from the cathode side to the anode side of the plate due to electrochemical oxidation (corrosion) of the surface of the graphite. Oxidation makes the graphite hydrophilic which results in it being wet by the acid electrolyte.
The thermally purified large flake graphites currently used in separator plate assemblies have two benefits relative to spherical graphites previously used. First, the low ratio of edge planes to basal planes with the large graphite flakes results in a very low corrosion rate compared to spherical graphite. Second, the current manufacturing process also results in the graphite flakes being preferentially aligned perpendicular to the thru-plane direction. This results in a very high tortuosity to thru-plane acid penetration which further impedes acid penetration into the plate.
Thermoplastics such as FEP are formed into tubing, film, sheets and insulating coatings on electrical wiring by using melt extrusion processes. These materials are typically 100% FEP. In some applications a few % fillers are added to color the products or affect their electrical properties. The extruder typically has multiple heating zones. The temperatures of these zones, according to DuPont, are typically 330-370° C. which is significantly higher than the melting point of a typical FEP which is about 260° C. The FEP exits the extruder in a molten, entirely liquid state and at ambient pressure. Various approaches are used to cool the formed article to solidify the FEP and to control the dimensions of the extruded article. FEP tubing, for example, is drawn over a mandrel while immersed in a water bath thus setting the diameter and solidifying the FEP. In one example manufacturing method, FEP film is calendared with chilled rolls to set its thickness and solidify the FEP.